1. Field of the Invention
This invention relates to an injection desulfurizing agent suitable for use in injection desulfurization treatment of hot metal.
2. Description of the Prior Art
Steel having a good quality has recently been strongly required. Particularly, sulfur in steel conspicuously deteriorates steel products in mechanical property, and hence, it is desirous to make the sulfur content in the steel as low as possible. To meet the requirement, it has been the common practice to subject the desulfurization treatment to the hot metal bath.
The desulfurizing agent suitable for the desulfurization of the hot metal has been investigated for long. At present, desulfurizing agents mainly consisting of calcium carbide (CaC.sub.2) are widely used owing to the reasons that such desulfurizing agents are high in reaction efficiency and are relatively easy in removing slag therefrom after the desulfurization treatment.
The calcium carbide, however, consumes much amount of electric power in the course of manufacture and hence is expensive. In addition, the use of the calcium carbide has the disadvantage that there is a risk of acetylene gas being produced during handling. As a cheaper desulfurizing agent, sodas such as sodium carbonate have also been used. These sodas tend to produce white smoke during the desulfurization treatment and hence is troublesome in operation. In addition, these sodas increase a melting loss of the refractory lining of a desulfurization treating vessel. Thus, such desulfurizing agent has not widely been used.
It has also been known that lime is considerably less expensive than carbide and sodas and has a sufficiently large desulfurizing capacity. Thus, it has long been tried to use the lime as the desulfurizing agent. The quick lime has the advantage that acetylene and white smoke are not generated, but has the important drawback that the desulfurization reaction is slow. As a result, the addition of much amount of quick lime has been required in order to make the desulfurization rate large. The use of such much amount of quick lime ensures an increase of the cost of the desulfurizing agent if compared with that of the carbide or sodas even though the desulfurizing agent per se is less expensive. In addition, the use of such much amount of lime induces the temperature drop during the desulfurization treatment and also increases the loss of hot metal into resulting slag. Thus, the lime is not used in practice.
Many methods of desulfurizing hot metal have also been investigated and use in practice. Recently, a method of injecting desulfurizing agent together with a carrier gas into the pig iron bath in a torpedo car through a lance has often been used.
This method has the advantage that a large amount of molten bath can be treated within a short time. But, the desulfurization reaction occurs within a short time elapsed from the injecting of the desulfurizing agent to the rising up thereof to the surface of the hot metal bath, and as a result, use must be made of a desulfurizing agent having a high reaction rate. Lime having a low reaction rate is not suitable for use as the desulfurizing agent for injection.
However, the lime is less expensive, easy in handling and not dangerous and hence has been appraised as a useful desulfurizing agent.
In order to improve the reaction efficiency of the lime used for the injection desulfurization, a method of making the particle diameter of the lime powders small so as to increase the interfacial reaction area or a method of adding a small amount of fluorspar or the like to the lime so as to make a portion of the lime low in its melting point, has been proposed.